Metallic antenna arrays fabricated on high resistivity silicon are used to localize and enhance the incident THz field resulting in high electric field pulses with peak electric field strength reaching several MV/cm on the silicon surface near the antenna tips. In such high electric field strengths high density of carriers are generated in silicon through impact ionization process. The high density of generated carriers induces a change of refractive index in silicon. By measuring the change of reflectivity of tightly focused 800 nm light, the local density of free carriers near the antenna tips is measured. Using the NIR probing technique, we observed that the density of carriers increases by over 8 orders of magnitude in a time duration of approximately 500 fs with an incident THz pulse of peak electric field strength 700 kV/cm. This shows that a single impact ionization process is happening in a time duration of less than 20 fs. The measurement is repeated by exciting the sample with an optical pump beam at a wavelength of 400 nm. The optical pump sets the initial free carrier density before the THz-induced impact ionization. The measurements show that the carrier generation mechanism depends on the initial free carrier density which confirms that the carrier generation mechanism is impact ionization, rather than the alternative carrier generation mechanism in high electric field, i.e. Zener tunneling. Finally this technique can be extended to investigate carrier dynamics in other semiconductors.
A theoretical study of photonic liquid crystal fibers (PLCFs) is presented. Applied numerical approach is based on fully-vectorial plane wave method for the calculation of modal structure of photonic crystal fiber. Used method allows for including material anisotropy that is essential in numerical simulations of the light propagation in photonic liquid crystal fibers. Influence of such parameters as molecules ordering and orientation on the photonic band gaps location and on the propagation constant values of guided modes are studied. It is shown that the guiding mechanism can be easily switched between index guiding and bandgap guiding simply by changing temperature. Moreover, the hybrid guiding for different polarizations can be achieved by applying external electric field perpendicular to the propagation direction.